1. Field of the Invention
This invention relates to a urea-formaldehyde resin composition, to methods of manufacturing the resin and using it, and to products prepared using the resin. More particularly, the invention relates to a urea-formaldehyde resin composition which is resistant to hydrolysis, cures quickly without smoking, and exhibits low formaldehyde emission. The resin is useful as a binder for making glass fiber mats and metal acid salt briquettes or composites.
2. Description of Related Art
Often, products containing fibrous or particulate materials are manufactured by binding the materials together by use of a binding composition. Such compositions typically are known as "binders".
Binders not only retain the fibrous or particulate materials in the desired orientation or shape, but also impart physical characteristics to the product. For example, mats of glass fibers bound together have a variety of forms and uses, such as support sheets for vinyl and other types of composite flooring, roofing shingles, or siding. Various types of binders are also used in the art to maintain the physical integrity of ferrous sulfate briquettes used in, for example, deodorizing applications and in fashioning other types of composites.
Preferred characteristics of binders used to bind glass fibers may be different from preferred characteristics of binders used to bind particulates. For example, it may be desirable to utilize a binder that is soluble in a preselected solvent so that the bound material is released into that solvent. It may be desirable to make the binder freely soluble in some solvent, affording quick release of the bound material, or to make the binder only sparingly soluble, thus delaying release of the material. Because the characteristics exhibited by the binder contribute to the overall characteristics of the product, the binder must be carefully selected.
Unpleasant odors and toxic gases emanate from various daily living environments and various facilities such as cattle farms, poultry farms and sewage disposal plants. Such odors are caused by acidic malodorous gases, such as hydrogen sulfide, as well as alkaline malodorous gases, such as ammonia. Various compounds such as ferrous sulfate, aluminum sulfate, zinc sulfate, zirconium oxide, zirconium phosphate, and titanium oxide are known to remove the bad odors of ammonium. Zinc oxide, magnesium oxide, iron oxide and iron hydroxide, while not very good at removing gaseous ammonia, have been used to eliminate hydrogen sulfide.
U.S. Pat. No. 4,735,972 discloses the incorporation of ferrous sulfate, aluminum sulfate, potassium aluminate sulfate, sodium aluminum sulfate or zinc sulfate into a thermoplastic resins to form a composition having deodorizing properties which can be molded into articles, such as bag for garbage disposal.
Published PCT Application No. 92/16461 discloses blocks of solid material made with synthetic resins containing at least formaldehyde and ferrous sulfate. The blocks may be reinforced with, for example, wood chips or fibers, cellulose fibers, and or acid peat. Gases collect on and are held by the peat, causing the blocks to float. A urea-formaldehyde resin having a U:F molar ratio between 1:1.1 and 1:1.5 is disclosed for use. The blocks are placed in, for example, basins situated under animals in breeding areas and float on the animal sewage surface where the ferrous sulfate is liberated gradually from the block to neutralize free ammonia by forming ammonia-ferro-sulfate and partially deodorize the sewage.
Typical binders used to bind glass fiber mats include urea-formaldehyde resins, phenolic resins, bone glue, polyvinyl alcohols, and latexes. These binder materials are impregnated directly into the fibrous mat and set or cured to provide the desired integrity for the glass fibers. The most widely used binder is urea-formaldehyde because it is inexpensive.
Glass fibers also have been used by themselves and in combination with other types of fibers in the production of paper-like sheet materials. Glass fibers have been used as such a supplemental fiber in specialty, synthetic, fiberboard, pulp, and composite papers, and are finding a use in glass fiber paper, a substitute for papers made of asbestos fiber. Also, there has been and continues to be increasing use of a nonwoven, sheet-like mat of glass fibers (particularly chopped glass fibers or strands, and combinations thereof) as a replacement for organic felts such as cellulose mats in roofing shingles and buildup roofing systems (BUR systems).
Use of the glass fiber mats in the roofing industry provides several advantages. These advantages include: reduction in the amount of asphalt necessary for the roofing products, reduction in the weight of the roofing products, increased production rates for producing roofing products, superior rot resistance, longer product life, and improved fire ratings. These nonwoven, sheet-like mats usually are produced in a process in which glass fibers (chopped fibers, chopped fiber strands, strands, and combinations thereon are dispersed in an aqueous medium and formed into a mat. The nonwoven, sheet-like mat product is produced by contacting the mat of glass fibers with a polymeric binder. An example of such a process is the "wet-laid process". Descriptions of the wet-laid process may be found in a number of U.S. patents, including U.S. Pat. Nos. 2,906,660, 3,012,929, 3,050,427, 3,103,461, 3,228,825, 3,760,458, 3,766,003, 3,838,995 and 3,905,067, all of the teachings of which are incorporated herein by reference.
The wet-laid process involves forming, usually with agitation in a mixing tank, an aqueous slurry of glass fibers, typically chopped fibers or chopped strands of suitable length and diameter. Other forms of glass fibers, such as continuous strands, also may be used. Generally, fibers having a length of about inch to 3 inches and a diameter of about 3 to 20 microns are used. Each bundle may contain from about 20 to 300, or more, of such fibers, which may be sized or unsized, wet or dry, as long as they can be suitably dispersed in an aqueous dispersant-containing medium. The bundles are added to the medium to form an aqueous slurry. Any suitable dispersant known in the art, e.g., polyacrylamide, hydroxyethyl cellulose, ethoxylated amines, and amine oxides, may be used. The dispersant is employed in relatively small amounts, e.g. 0.2-10 parts in 10,000 parts of water.
The fiber slurry is agitated to form a workable, well-dispersed slurry having a suitable consistency. The aqueous slurry, often referred to as slush, is processed into the wet-laid, nonwoven, sheet-like mat by such machines as cylinder or Fourdrinier machines. More technologically-advanced machinery, such as the StevensFormer, RotoFormer, InverFormer, and the VertiFormer machines, also are used. The slush is deposited in some manner from a head box onto a moving wire screen or onto the surface of a moving wire-covered cylinder. On route to the screen, the dispersion usually is diluted with water to a lower fiber concentration. The slurry on the screen or cylinder is processed into the sheet-like that by the removal of water, usually by suction and/or vacuum devices, and the application of a polymeric binder. Binder composition is applied by soaking the mat in an excess of binder solution, or by coating the mat surface by means of a binder applicator, for example, by roller or spray. Suction devices often are utilized for further removal of water and excess binder and to ensure a thorough application of binder.
Thus-incorporated binder is cured, typically in an oven at elevated temperatures. Generally, a temperature of at least about 200.degree. C. is used during curing. Normally, this heat treatment alone will effect curing. Catalytic curing, such as is accomplished with an acid catalyst (for example, ammonium chloride or p-toluene sulfonic acid), generally is a less desirable alternative.
Typically, when urea-formaldehyde resins are used as a binder component they release formaldehyde into the environment during cure or formaldehyde is released from the cured resin, particularly when the cured resin is exposed to acidic environments. Such release is undesirable, particularly in enclosed atmospheric environments. In such environments, formaldehyde is inhaled and comes into contact with the eyes, the mouth, and other parts of the body. Formaldehyde is malodorous and causes human and animal illness.
Various techniques have been utilized to reduce formaldehyde release from urea-formaldehyde resins. Use of formaldehyde scavenger and various methods for resin formulation, including addition of urea as a reactant late in the resin formation reaction, are techniques used to reduce formaldehyde emission. However, use of formaldehyde scavenger often is undesirable, not only because of the additional cost, but also because it affects the characteristics, or properties, of the resin. For example, using ammonia as formaldehyde scavenger reduces the resistance of the cured polymer to hydrolysis (degradation). Later addition of urea to reduce free formaldehyde concentration in the resin may yield a resin that must be cured at a relatively low rate to avoid smoking and polymer stability also can be adversely effected.
U.S. Pat. No. 2,260,033 describes a method which purportedly reduces the amount of free formaldehyde in a urea-formaldehyde resin. In the disclosed process, triethanolamine is added to a mixture of urea and formaldehyde having a 1:1 to 1.5:1 formaldehyde to urea mole ratio in an amount sufficient to neutralize its pH. The mixture is then reacted at 30.degree. C. The resin is used to make molded objects, laminated material and films.
U.S. Pat. No. 2,626,251 describes the preparation of a water soluble, cationic urea-formaldehyde resin. The resin is disclosed as having a high degree of water resistance when cured and is suggested for use in textile applications and for adding wet strength to paper. The preferred resin is prepared by initially reacting urea and formaldehyde at a formaldehyde to urea mole ratio of at least 2.0 but less than 3.0 together with triethanolamine in a urea to triethanolamine mole ratio of 2.0 to not more than 3.0. The resin thus-formed then is made cationic by acidifying it to a pH below 2.5, and preferably at least 1.5, with a strong inorganic acid such as hydrochloric, sulfuric or nitric, followed by prompt neutralization to a pH of 6 to 7. A pH above 7 is discouraged as this is said to retard the cure of the resin.
U.S. Pat. No. 3,882,462 to Pearson, describes a urea-formaldehyde resin prepared by reacting sequentially aqueous formaldehyde, a catalyzing acid, triethanolamine and urea. The aqueous resin is taught for use in coatings, adhesives and textile finishes. The preferred resin is prepared using 30 moles of formaldehyde, 2 moles of acid, preferably phosphoric acid, 2 moles of triethanolamine and 12 moles of urea. The various reactants are said to react, without applied heat, as rapidly as the materials are mixed together. In U.S. Pat. No. 4,119,598, said to be an improvement on the '462 patent, the formaldehyde, urea and triethanolamine are mixed before addition of the acid and the molar quantities, based on about 30 moles of formaldehyde, are changed to 0.13 mole acid, 1.6 mole triethanolamine and 9.9 moles urea. In yet another improvement patent, U.S. Pat. No. 4,370,442, melamine is included in the reaction mix to expand the resin's water dilutability and storage stability. Finally, in U.S. Pat. No. 4,663,239, Pearson describes including ammonium hydroxide, ammonium chloride and ammonium formate in the composition to reduce formaldehyde emissions.
U.S. Pat. No. 4,492,699 describes a urea-formaldehyde resin adhesive for wood composites, such as particle board, characterized by slow formaldehyde emission. The patent indicates that by increasing the level of methylene linkages in the resin. instead of dimethylene ether linkages and methylol end groups, hydrolytic degradation, which contributes to increased formaldehyde emission, is reduced. To accomplish this goal, the resin is prepared in a process having two stages of condensation and two stages of methylolation. In a first condensation stage, urea is added to a highly acidic formaldehyde solution (pH of 0.5 to 2.5) at a formaldehyde to urea mole ratio of 2.5 to 4.0. The initial stage is very exothermic and proceeds without the application of heat. The reaction can be controlled to a temperature in the range of 50.degree. C. to 99.degree. C. by adding the urea incrementally. Thereafter, the resin solution is neutralized and additional urea is added. Triethanolamine is one of several bases mentioned for neutralizing the resin and a combination of sodium hydroxide and triethanolamine is preferred. After the second stage, the formaldehyde to urea mole ratio is within the range of 1.5: 1.0 to 2.5: 1.0. The second step is conducted at a temperature of 50.degree. C. to 80.degree. C. to permit methylolation to proceed slowly. The resin then is switched again to an acidic pH, heated to reflux and reacted to a desired viscosity. Finally, the resin is neutralized to slight alkalinity (pH of 7.3-7.5) and additional urea is added to provide a cumulative formaldehyde to urea mole ratio of 1.1:1.0 to 2.3:1.0. Methylolation is said to thereafter proceed during storage. Thereafter, the resin is cured to an infusible state during use by adding ammonium chloride and heating at 115.degree. C. for 15 minutes.
U.S. Pat. No. 4,968,773 describes preparing a urea-formaldehyde resin purportedly having a low extractable formaldehyde content by first methylolating urea under alkaline conditions (pH of 6-11) at a formaldehyde to urea mole ratio within the range of 2:1 to 3:1, followed by condensation at a low (acid) pH (pH of 0.5-3.5), then neutralizing the resin (pH of 6.5-9) and adding additional urea to yield a final formaldehyde to urea mole ratio of 1.8:1 or less.